Brightness stabilizing control of a VF display

ABSTRACT

A VF display control apparatus operated directly from an automotive storage battery in which the display brightness variation is minimized by controlling the relationship between the anode and grid voltages in relation to the fluctuation of the battery voltage. The anodes of the display are operated substantially at the battery voltage, and the grid voltage is reduced in relation to the amount by which the anode (supply) voltage exceeds the nominal open-circuit terminal voltage of the battery.

This invention relates to the control of an automotive vacuumfluorescent (VF) display, and more particularly, to a method andapparatus for minimizing the display brightness variations which occurdue to variations in the supply voltage.

BACKGROUND OF THE INVENTION

Vacuum fluorescent (VF) displays are generally defined by an evacuatedenvelope enclosing one or more phosphored anodes arranged in a patternof desired light emission, a filament and a grid disposed between theanodes and filament. The filament is electrically heated at a relativelylow voltage to generate a cloud of electrons, and the grid is maintainedat a relatively high voltage to accelerate electrons onto any of theanodes which are also maintained at a relatively high voltage. Theanodes bombarded by electrons emit light due to the phosphor coating.

In automotive applications, the anodes, filament and grid are generallyreferenced to the storage battery, as shown in the PRIOR ART drawing ofFIG. 1. Referring to FIG. 1, the storage battery 10 is connected byignition switch 12 to a supply terminal T which, when referenced to thevehicle frame, is generally referred to as the ignition voltage or IGN.The VF display is generally designated by the reference numeral 14 andcomprises a filament 16, a grid 18 and a plurality of anode segments 20.The anode segments 20 are individually and selectively connected to theignition voltage IGN through an anode driver array 22 and a dimmingcircuit 24. The anode driver array 22 and a dimming circuit 24. Theanode driver array comprises a plurality of solid state switches 26which are individually controlled to define the pattern of desired lightemission, and the dimming circuit 24 comprises a solid state switch 28which is pulse-width-modulated to control the average anode voltage andtherefore the overall brightness of the display 14. A control of thissort is generally required for operator adjustment of the displaybrightness in night driving conditions. The grid 18 is maintainedsubstantially at the ignition voltage IGN through the resistor R_(g) andthe filament is energized at a relatively low potential via a droppingresistor R_(f) or a separate low voltage power supply (not shown). Whenmultiplexing is employed, a grid supply switch 30 may be provided foropen-circuiting the grid 18 to turn off the entire portion of thedisplay 14 situated under the grid.

A drawback of the above-described drive circuit is that the displaybrightness tends to vary with the terminal voltage of the battery 10. Incertain displays, brightness variations of 60% or more have beenobserved when the battery voltage is allowed to fluctuate over a 12-16volt range. The usual solution is to insert a regulated power supplybetween the battery and the display. This, of course, is quiteexpensive, especially if a switching regulator is required.

SUMMARY OF THE PRESENT INVENTION

The present invention is directed to an improved VF display controlapparatus operated directly from an automotive storage battery, whereinthe display brightness variation is minimized by controlling therelationship between the anode and grid voltages in relation to thefluctuation of the battery voltage. In essence, we have discovered thatthe brightness fluctuations of a VF display can be reduced orsubstantially eliminated over a range of supply voltages by driving theanode and grid such that the grid voltage varies in inverse relation tothat of the anode voltage.

In operation, the anode and filament voltages are ratiometricallyrelated to the battery voltage, and the grid is supplied with anindependently variable voltage intermediate that of the anode andfilament. In the illustrated embodiment, the anodes of the display areoperated substantially at the battery voltage, and the voltage suppliedto the grid is reduced in relation to the amount by which the anode(supply) voltage exceeds the nominal open-circuit terminal voltage ofthe battery. The voltage increase at the filament is relatively slightcompared to the voltage increase at the anode, and the reduced gridvoltage compensates for the higher anode-to-filament potentialdifference by reducing the grid-to-filament potential difference. As aresult, the anode is bombarded by fewer but more energetic electrons andthe display brightness tends to remain relatively constant. In amechanization of the illustrated embodiment, the overall displaybrightness variation over a supply voltage range of 12-16 volts wasreduced to less than 10%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a prior art control circuit for a VFdisplay.

FIG. 2 is a graph depicting the intensity of a VF display as a functionof the grid voltage for various anode voltages within the normal rangeof automotive battery voltage fluctuation.

FIG. 3 is a graph depicting the grid voltage vs. anode voltage requiredto maintain the brightness of the VF display of FIG. 2 substantiallyconstant over a range of battery voltages.

FIG. 4 is a circuit for mechanizing the relationship depicted in thegraph of FIG. 3.

FIG. 5 is a graph depicting the performance of the circuit of FIG. 3 interms of measured display brightness over the supply voltage range of12-16 volts.

DETAILED DESCRIPTION OF THE DRAWINGS

As indicated above, the prior control circuit of FIG. 1 exhibitssignificant display brightness variation due to supply voltagevariations. The characteristic graphs of FIG. 2 were generated as partof an analysis of this phenomenon. Referring to FIG. 2, the intensity orbrightness of a given VF display is plotted as a function of gridvoltage for various anode voltage values within the normal range offluctuation of automotive ignition voltage, the filament voltage beingmaintained substantially constant. In the conventional display drivecircuit of FIG. 1, the grid voltage generally follows the anode(ignition) voltage resulting in the indicated brightness fluctuations.

However, this invention recognizes that the display brightness can bemaintained substantially constant over a range of ignition voltages bycontrolling the relation between the anode and grid voltages along agiven constant brightness load line, as represented by the trace 32 inFIG. 2. The relation between the anode and grid voltages for thebrightness represented by the trace 32 is depicted by the trace 34 ofFIG. 3. Various traces similar to the trace 34 can be developed for anyvalue of constant brightness depicted in FIG. 2. Significantly, suchtraces define an inverse relation between the anode and grid voltages.

The constant intensity relationship described above in reference to thetraces 32 and 34 can be approximated with the grid drive circuit of FIG.4 to produce the brightness performance depicted in FIG. 5. Referring toFIG. 4, elements corresponding to those depicted in FIG. 1 have beenassigned the same reference numerals. Thus, the exciting current forfilament 16 is supplied from the ignition voltage IGN via droppingresistor R_(f), and the anodes 20 are selectively connected to theignition voltage IGN via the anode driver array 22 and the dimmingcircuit 24. However, the grid voltage is now controlled by the griddrive circuit designated generally by the reference numeral 40.

The grid drive circuit 40 comprises a first transistor 42 connecting theignition voltage IGN to the display grid 18 and a second transistor 44for limiting the conduction of transistor 42 when the ignition voltageIGN (and hence, the anode voltage) rises above a reference voltage V_(z)defined by the Zener diode 46. So long as the ignition voltage is lessthan or equal to the Zener voltage V_(z), the transistor 44 ismaintained in a nonconductive state by the pulldown resistor 48, and thetransistor 42 is maintained in a fully conductive state by the pullupresistor 50. In this state, the potential of grid 18 is maintainedapproximately one diode drop below the ignition (anode) voltage IGN.

When the ignition voltage rises above the Zener voltage V_(z), thetransistor 44 begins to conduct, diverting some of the base current oftransistor 42 to ground through the resistor 52. This causes transistor42 to operate in its linear region which increases the voltage dropacross its collector-emitter circuit and correspondingly decreases thevoltage applied to the grid 18 according to the relationship defined bythe broken trace 36 of FIG. 3.

In the conventional circuit of FIG. 1, increases in the supply (anode)voltage produce similar increases grid-to-filament voltage since thecorresponding increase in the filament voltage is relatively slight.This increases both electron flow and the energy level of the electronsat the anode and therefore increases the brightness of the emittedlight. With the control of this invention, however, increases in theanode voltage are accompanied by decreases in the grid voltage, therebyreducing the grid-to-filament voltage. As a result, the anode isbombarded by fewer, more energetic electrons and the display brightnesstends to remain relatively constant, as graphically depicted in FIG. 5,where the measured display brightness or intensity in FT-L is given as afunction of the ignition voltage IGN. As seen in the graph, theintensity variation is less than 10% over an ignition voltage range of12-16 volts, the range one would normally experience in the operation ofa motor vehicle.

While this invention has been described in reference to the illustratedembodiment, it will be recognized that various modifications will occurto those skilled in the art. In the illustrated embodiment, the nominalopen-circuit terminal voltage of the storage battery is chosen as abaseline operating point, above which the grid voltage is made todecrease with increasing anode voltage. However, the primary import ofthe present invention is that the brightness fluctuations of a VFdisplay can be reduced or substantially eliminated over a range ofsupply voltages by driving the anode and grid such that the grid voltagevaries in inverse relation to that of the anode voltage. Thus, it willbe understood that the scope of this invention is broader than theillustrated embodiment and is only limited by the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. In a display systemincluding a vacuum fluorescent tube in which electrons generated at afilament element and attracted by a grid element bombard a phosphoredanode element to emit light for display purposes, and control apparatusfor supplying operating voltages to the filament, grid and anodeelements from a variable voltage source, the improvement wherein:thevoltage supplied to the anode element follows the voltage of saidsource; and the voltage supplied to the grid element is varied ininverse relation to that of the voltage supplied to the anode element,thereby to reduce fluctuations in the brightness of the emitted lightdespite substantial variation of the source voltage.
 2. In a displaysystem including a vacuum fluorescent tube in which electrons generatedat a filament element and attracted by a grid element bombard aphosphored anode element to emit light for display purposes, and controlapparatus for supplying operating voltages to the filament, grid andanode elements from a source having a variable output voltage, theimprovement wherein:the anode element is operated substantially at theoutput voltage of said source; and the grid is operated at a voltagelower than the output voltage of said source and which varies in inverserelation thereto, at least when said output voltage exceeds a referencevoltage.
 3. The improvement of claim 2, wherein:the source includes anautomotive storage battery having a nominal open-circuit output voltage,the battery being adapted to be charged at voltages in excess of suchnominal open-circuit voltage; and the reference voltage substantiallycorresponds to said nominal open-circuit voltage.
 4. In a display systemincluding a vacuum fluorescent tube in which electrons generated at afilament element and attracted by a grid element bombard a phosphoredanode element to emit light for display purposes, control apparatus forsupplying operating voltages to the filament, grid and anode elementsfrom a source having a variable output voltage so as to minimize sourcevoltage related variations in the brightness of the emitted light,comprising:anode supply means connected between the source and the anodeelement for supplying an operating voltage to the anode element whichfollows the output voltage of said source; and grid supply meansconnected between the source and the grid element for supplying the gridelement with an operating voltage which varies in inverse relation tothe voltage supplied to the anode element.
 5. In a display systemincluding a vacuum fluorescent tube in which electrons generated at afilament element and attracted by a grid element bombard a phosphoredanode element to emit light for display purposes, control apparatus forsupplying operating voltages to the filament, grid and anode elementsfrom a source having a variable output voltage so as to minimize sourcevoltage related variations in the brightness of the emitted light,comprising:anode supply means connected between the source and the anodeelement for supplying an operating voltage to the anode element which issubstantially equal to the output voltage of said source; and gridsupply means connected between the source and the grid element forsupplying the grid element with an operating voltage which is lower thanthe voltage supplied to said anode element and which varies in inverserelation thereto, at least when said output voltage exceeds a referencevoltage.
 6. The apparatus set forth in claim 5, wherein:the sourceincludes an automotive storage battery having a nominal open-circuitoutput voltage, the battery being adapted to be charged at voltages inexcess of such nominal open-circuit voltage; and the reference voltagesubstantially corresponds to said nominal open-circuit voltage.
 7. In adisplay system including a vacuum fluorescent tube in which electronsgenerated at a filament element and attracted by a grid element bombarda phosphored anode element to emit light for display purposes, controlapparatus for supplying operating voltages to the filament, grid andanode elements from a source having a variable output voltage so as tominimize source voltage related variations in the brightness of theemitted light, comprising:anode supply means connected between thesource and the anode element for supplying the anode element with anoperating voltage substantially equal to the output voltage of saidsource; filament supply means connected between the source and thefilament element for supplying the filament element with a relativelylow operating voltage ratiometrically related to the output voltage ofsaid source; grid supply means connected between the source and the gridelement for supplying the grid element with an operating voltageintermediate the operating voltages supplied to said anode and filamentelements, at least when said output voltage is less than a referencevoltage; and brightness control means for reducing the operating voltagesupplied to the grid element by said grid supply means in relation tothe amount by which the operating voltage supplied to the anode elementexceeds said reference voltage.
 8. The apparatus set forth in claim 7,wherein:the source includes an automotive storage battery having anominal open-circuit output voltage, the battery being adapted to becharged at voltages in excess of such nominal open-circuit voltage; andthe reference voltage substantially corresponds to said nominalopen-circuit voltage.
 9. In a display system including a vacuumfluorescent tube in which electrons generated at a filament element andattracted by a grid element bombard a phosphored anode element to emitlight for display purposes, control apparatus for supplying operatingvoltages to the filament, grid and anode elements from a source having avariable output voltage so as to minimize source voltage relatedvariations in the brightness of the emitted light,comprising:ratiometric supply means connecting the source to the anodeand filament elements for supplying the anode and filament elements withrelatively high and low operating voltages, respectively, whichratiometrically follow the output voltage of said source; and gridsupply means connected between the source and the grid element forsupplying the grid element with an independently variable operatingvoltage intermediate the operating voltages of said anode and filamentelements such that increases in the anode voltage are accompanied bydecreases in the grid voltage to effect a lower voltage differencebetween the filament and grid elements, whereby the anode is bombardedby fewer electrons and the brightness of the emitted light tends toremain relatively constant.